Rotating fluid pumps and motors, and especially rotating piston pumps and motors, are well known in the prior art. The pumps are often used in hydraulic systems to provide fluid power to components such as hydraulic cylinders and rotary actuators. The fluid motors are often used to provide mechanical power to components such as electrical generators.
Rotary piston pumps and motors are characterized by a rotating piston group and associated swash plate. In a typical arrangement, a plurality of pistons are carried in a barrel that is rotatably mounted in a case. The pistons project from an axial end of the barrel for engagement with the swash plate. In a pump, the barrel is rotatably driven as by means of an input shaft coupled thereto. When the barrel rotates, the pistons are reciprocated back and forth within respective cylinders in the barrel through interaction with the swash plate that is tilted to the axis of the barrel. The piston cylinders are sequentially connected to inlet and outlet passages such that the reciprocating movement of the pistons draws fluid into the cylinder from an inlet passage and pushes the fluid out through an outlet passage.
A rotary piston motor operates generally in reverse manner. The cylinders in the barrel are sequentially connected to a hydraulic fluid supply and return. When the pistons are axially driven by fluid pressure against the tilted swash plate, a reactionary torque is applied to the barrel thereby to rotate the barrel and in turn an output shaft.
In such pumps and motors, windage losses arise when the pump/motor case is filled with hydraulic fluid. Hydraulic fluid may leak from the rotating piston group thereby pressurizing the case and causing fluid flow out of a case drain port and back to the reservoir of the hydraulic system. It is desirable to scavenge the pump/motor case to reduce windage losses and thereby increase efficiency of the pump or motor. Although the benefits of scavenging the case have long been known, this has not been successfully implemented in a closed hydraulic system where the case is not directly vented to atmosphere. Accordingly, there exists a need for a pump and/or motor that obtains the benefits of a scavenged case in a closed hydraulic system where the case is not directly vented to atmosphere. Moreover, the solution should be such that air is not ingested into the hydraulic system due to a dynamic seal or seals being subjected to low scavenge pressure in the case.